1. Field
The present invention is directed toward an improved ultrasonic cleaner, and particularly one providing amplitude and frequency modulation of ultrasonic vibrations.
2. State of the Art
In ultrasonic cleaners, articles to be cleaned are placed in a liquid bath in a cleaning tank. A piezoelectric transducer is mounted to the tank to convert electrical energy into mechanical vibrations in the water. An ultrasonic signal generated by a driver circuit energizes the transducers to vibrate at their prescribed frequency, which is preferably a resonant frequency of the particular transducers used.
It is known that the efficiency of ultrasonic cleaners can be improved by modulating the amplitude or the frequency of the ultrasonic signal presented to the transducers. Without modulation, standing waves may occur within the tank, allowing for uneven cleaning.
Amplitude modulation is typically accomplished by deriving a modulation signal from the frequency of the AC line voltage that powers the ultrasonic cleaner. This modulation signal is presented to the transducers to modulate the amplitude of the ultrasonic signal within the "envelope" of the derived modulation signal. In the United States, 120 volt, 60 Hz, AC line voltage is common. In Europe, 240 volt, 50 Hz AC line voltage is the norm. The amplitude modulation signal is typically derived by a full wave rectification of the power line voltage. Thus, in the United States, amplitude modulation typically occurs within a 120 Hz envelope, while in Europe, amplitude modulation occurs within a 100 Hz envelope.
The modulation envelope provided by the AC line voltage is generally sinusoidal. When the sinusoidal signal is full wave rectified, the lower half of the sine wave signal is "flipped up" to the positive voltage side. As viewed on an oscilloscope, the amplitude envelope signal looks like a series of "bumps" with zero volt nodes between the bumps.
The modulation signals of such devices are chosen because they are easily derived from available line voltage. However, it appears that neither the frequency nor the shape of modulation signals derived from common line voltages are optimum for achieving maximum efficiency in ultrasonic cleaners. There remains a need for an ultrasonic cleaner having a drive circuit that provides for amplitude and frequency modulation and signal shaping chosen to maximize cleaning effectiveness and transducer output, independent from the frequency and signal shape of available line voltage.